A suspension pump with a built-in gas evacuation pump of the type described above is previously known through U.S. Pat. No. 4,776,758, however, the gas evacuation pump functions only as a vacuum pump. Since the vacuum pump and the suspension pump are driven by a common shaft the vacuum pump cannot be disconnected in those cases when no gas is separated out from the suspension- The vacuum pump must therefore operate under extremely varying conditions, both in different installations and in one and the same installation. Suspension pumps in down pipes containing pulp of medium consistency at a low level and much air require a vacuum pump with high capacity in order to be able to evacuate sufficiently large quantities of air at low pressure on the inlet side of the suspension pump. Suspension pumps operating with high pulp levels on the inlet side, e.g. a suspension pump installed at the bottom of a storage tower, need only be de-aired when the pulp level drops below 15 m. At higher levels the vacuum pump works towards a closed outlet and contributes to unnecessary power consumption, however other problems may arise as will be clear below.
When the level of pulp in a storage tower is high, normally above 15 m, the pressure at the inlet to the pump is high, e.g. 2 bar, and no air will therefore be separated out from the pulp. The pressure inside the pump, at the centre of the impeller is considerably above atmospheric pressure. Under such operating conditions--high inlet pressure and no air separation--the vacuum pump will be filled with pulp and its outlet must be closed. When the vacuum is operating against a closed outlet in this way, and is filled with pulp, considerable pressure variations occur which lead to cavitation and mechanical stress.
When the level of the pulp is mid-way in the storage tower, usually 7-15 m, the pressure at the inlet to the pump will be correspondingly lower, e.g. 1 bar, and some of the air will therefore be separated out in the pump, disturbing the pumping process. The pressure at the centre of the impeller is still above atmospheric pressure, and it is difficult to evacuate air without the fibre suspension being pressed out at the same time. The outlet from the vacuum pump must therefore be throttled. However, such throttling causes the outlet to become blocked by fibres and to solve this problem the outlet is flushed clean with water from a pipe which is connected to the outlet before the throttle valve.
When the level of the pulp is low in the storage tower, usually below 7 m, the pressure at the inlet to the pump will be correspondingly lower, atmospheric pressure and below, so that large and varying amounts of air in the pulp will be separated out in the pump, disturbing the pumping process. The pressure at the centre of the impeller is now below atmospheric pressure, and the air is withdrawn by means of the vacuum pump. Since the amounts of air separated out vary from one time to another, the pressure will vary. Theefore, in order to maintain the pressure constant, make-up air is added via a conduit provided with a pressure-sensitive valve which is set at predetermined negative pressure, e.g. -0,4 bar. Furthermore, the built-in vacuum pump must be dimensioned for a capacity corresponding to the worst possible operating conditions, i.e. it must be able to evacuate the largest possible amounts of air which can be separated out from the pulp at low pressures.
It is clear from the above that there are three different operating conditions for the pump with built-in vacuum pump, that can appear in one and the same pump installation, namely:
1. Low inlet pressure, below atmospheric pressure, and large amounts of air which are separated out in the pump.
2. Medium inlet pressure, above atmospheric pressure, and air separation in the pump.
3. High inlet pressure, above atmospheric pressure, and no air separation in the pump.
It is recognized that in the second and third operating conditions the built-in vacuum pump results in certain drawbacks. A problem also exists in the first operating condition since make-up air must be added in order to maintain constant pressure in dependence on the varying amounts of air separated out in front of the impeller. There is also considerable risk of the channel system for the make-up air being out of function due to pulp having penetrated into the channel system during the first and second operating conditions so that the vacuum pump must be dismantled to allow cleaning of the channel system. Obviously this entails interruptions in operation.